This invention relates to reduction of zirconium or hafnium chloride after (or in combination with) separation of hafnium from zirconium and in particular relates to electrochemically-metallothermically reducing zirconium or hafnium in a molten salt bath.
Molten (fused) salt electrochemical (electrolytic) processes for deposition of metal on one electrode (with evolution of chlorine gas at the other electrode) are known in the art. U.S. Pat. No. 3,764,493 to Nicks et al., and U.S. Pat. No. 4,670,121 to Ginatta et al. are examples of such processes.
Naturally occurring zirconium ores generally contain from 1 to 3 percent hafnium oxide relative to zirconium oxide. In order that the zirconium metal be acceptable as a nuclear reactor material, the hafnium content must first be reduced to low levels, due to the high neutron absorption cross section of hafnium. This separation process is difficult due to the extreme chemical similarity of the two elements. A number of techniques have been explored to accomplish this separation, with the technique currently in use in the United States involving liquid-liquid extraction of aqueous zirconyl chloride thiocyanate complex solution using methyl isobutyl ketone, generally as described in U.S. Pat. No. 2,938,769, issued to Overholser on May 31, 1960, with the removal of iron impurities prior to solvent extraction generally as described in U.S. Pat. No. 3,006,719, issued to Miller on Oct. 31, 1961.
Several other processes have been suggested for separation of the zirconium-hafnium tetrachloride (Zr,Hf)Cl.sub.4 generated from the ore by carbochlorination. U.S. Pat. No. 2,852,446, issued to Bromberg on Sept. 16, 1958, describes a high pressure distillation process where the pressure, rather than a solvent, provides for a liquid phase. U.S. Pat. No. 2,816,814 issued to Plucknett on Dec. 17, 1957, describes extractive distillation for separation of the tetrachlorides using a stannous chloride solvent. U.S. Pat. No. 4,021,531 issued to Besson on Apr. 3, 1977, utilizes extractive distillation with an alkali metal chloride and aluminum (or iron) chloride mixture as the solvent. Processes for zirconium-hafnium separation are described in U.S. Pat. Nos. 4,737,244, 4,749,448 issued to McLaughlin et al. and to Stoltz et al., provide for zirconium-hafnium separation by extractive distillation with the molten solvent containing zinc chloride and a viscosity reducer. Another separation process involves fractionation of the chemical complex formed by the reaction of (Zr,Hf)Cl.sub.4 with phosphorus oxychloride (POCl.sub.3). This technique was patented in 1926 by van Arkel and de Boer (U.S. Pat. No. 1,582,860), and was based on the approximately 5.degree. C. boiling point difference between the hafnium and zirconium complex pseudoazeotropes, having the nominal compositions (Zr,Hf)Cl.sub.4.(2/3)POCl.sub.3. Despite an extensive investment in time and money, the liquid-liquid extraction described in the above-mentioned U.S. Pat. No. 2,938,769 of Overholser remains the only commercially utilized process for zirconium-hafnium separation in the United States today.
Zirconium, hafnium and titanium are commonly reduced from the chloride by means of a reducing metal such as magnesium or sodium. At the present time the commercial processes are batch-type processes. U.S. Pat. No. 3,966,460, for example, describes a process of introducing zirconium tetrachloride vapor onto molten magnesium, with the zirconium being reduced and traveling through the magnesium layer to the bottom of the reactor and with the by-product magnesium chloride being periodically removed.
In commercial processes, a portion of the by-product salt (e.g. magnesium chloride) is removed manually after the batch has been completed and cooled, and the remainder of the salt and the remaining excess reducing metal is removed in a distillation or leaching process.
Modifications to the reduction process have been suggested in many U.S. Patents, including U.S. Pat. Nos. 4,511,399, 4,556,420, 4,613,366, 4,637,831, and 4,668,287, assigned to the same assignee. A high temperature process using zirconium tetrachloride as a part of a molten salt bath in which zirconium is reduced from the chloride to the metal (molten salt systems mentioned were potassium-zirconium chlorides and sodium-zirconium chlorides) is suggested in U.S. Pat. No. 2,214,211 to Von Zeppelin et al. A relatively high temperature process using zirconium tetrachloride as a part of a molten salt bath and introducing magnesium to reduce zirconium from the chloride to the metal (with external electrolytic reduction of magnesium from the chloride to the metal, to recycle magnesium) is suggested in U.S. Pat. No. 4,285,724 to Becker et al. Another high temperature process using zirconium tetrachloride as a part of a molten salt bath and which introduces sodium-magnesium alloy to reduce zirconium from the chloride to the metal (with a molten salt of magnesium chloride and sodium chloride) is suggested in U.S. Pat. No. 2,942,969 to Doyle. Using zirconium tetrachloride as a part of a molten salt bath and preferably introducing aluminum (but possibly magnesium) to reduce zirconium from the chloride to the metal, generally with the aluminum being introduced dissolved in molten zinc is taught by Megy in U.S. Pat. No. 4,127,409. Electrolytic-refining (metal in, metal out purification, rather than reduction from the chloride) processes are suggested in U.S. Pat. Nos. 2,905,613 and 2,920,027.
Direct electrolysis of zirconium has been reported in all-chloride molten salt systems, in mixed chloride-fluoride systems, and in all fluoride systems (Martinez et al, Metallurgical Transactions, Vol. 3, Feb. 1972-571; Mellors et al, J. of the Electrochemical Soc., Jan. 1966-60). All-metallic deposits were obtained from fluoride-containing baths (e.g. at 800.degree. C. using sodium fluorozirconate), but the efforts to plate out of all-chloride baths always produced a significant amount of subchlorides.